This invention relates to a new process for the preparation of liquid storable isocyanate mixture containing carbodiimide and/or uretone imine groups, to the mixtures obtainable by this process, and to their use in the preparation of polyurethane plastics, preferably polyurethane foams.
Isocyanate mixtures containing carbodiimide and/or uretone imine groups can be produced particularly easily by the basic method according to U.S. Pat. No. 2,853,473 using catalysts of the phospholine oxide series, which are by far the most effective for this purpose. Although this high catalytic activity is very desirable for activating the carbodiimidization reaction under moderate temperature conditions, no process for effectively terminating the phospholine oxide catalysis without limitations has been available. Effective termination such as this is of considerable industrial interest, particularly when liquid storable isocyanate mixtures containing certain numbers of carbodiimide and uretone imine groups are required, whether for liquefying 4,4'-diisocyanatodiphenylmethane or for preparing storable polyisocyanate mixtures having a fixed NCO content, for example, for the preparation of carbodiimide-containing foams.
Because of the considerable industrial interest, there has of course been no shortage of attempts to find a method of effectively terminating phospholine oxide catalysis.
Thus, anhydrous hydrochloric acid is said to have an adequate deactivating effect. Angew. Chem., 93, 859 (1981). German Offenlegungsschrift 2,614,323 discloses thionyl chloride as a preferred "deactivator".
According to German Offenlegungsschrift 2,537,685, these and all other additives which react with the catalyst with adduct or salt formation are suitable as deactivators. On the other hand, it is pointed out elsewhere in the same German Offenlegungsschrift 2,537,685 that such deactivation originally seemed to have little prospect of success "because it was known from DE-OS 2 245 634 that the adducts in question are themselves carbodiimidization catalysts for isocyanates". Accordingly, the use of very small quantities of catalyst (in the ppb to ppm range) and large excesses of deactivator are recommended. Quite apart from the contaminating and activity-reducing effect of such quantities of deactivator, this measure is also inadequate for obtaining long storage life without further, albeit greatly reduced, elimination of CO.sub.2 and the resulting increase in viscosity, because this highly active catalyst (or corresponding adducts) as a "genuine" catalyst retains prohibitive residual activity, even in very highly diluted form. Naturally, this residual activity remains greater when more phospholine oxide catalyst must be used to activate the carbodiimidization reaction. This behavior applies, for example, to polyisocyanate mixtures of the diphenylmethane series that contain fractions of sump product and, hence, reaction-inhibiting impurities.
Accordingly, the problem addressed by the present invention was to provide a new process for the preparation of liquid storable isocyanate mixtures containing carbodiimide and/or uretone imine groups which would help to eliminate the deficiencies mentioned above. Surprisingly, this problem has been solved by the invention described in detail hereinafter.